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9022 



Bureau of Mines Information Circular/1985 



Field Trials of a Portable Microseismic 
Processor Recorder 

By John P. Coughlin and Clinton D. Sines 




UNITED STATES DEPARTMENT OF THE INTERIOR 



I75J 

4f/NES 75TH A^ 



Information Circular 9022 

Field Trials of a Portable Microseismic 
Processor Recorder 

By John P. Coughlin and Clinton D. Sines 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Library of Congress Cataloging in Publication Data: 



Coughlin, John P 

Field trials of a portable microseismic processor recorder. 

(Information circular / United States Department of the Interior, 
Bureau of Mines ; 9022) 

Bibliography: p. 8. 

Supt. of Docs, no.: I 28.27:9022. 

1. Mine safety — Equipment and supplies— Testing. 2. Seismometers — 
Testing. I. Sines, Clinton D. II. Title. III. Series: Information cir- 
cular (United States. Bureau of Mines) ; 9022. 



TN295.U4 622s [622\8] 84-23809 






<ax. 



V>» 



^ CONTENTS 



'V Page 

Abstract 1 

Introduction 2 

General description of portable MPR 2 

Ease of installation and portability 4 

Data storage and computer link. 4 

Suppression of mine drilling noise 4 

Top panel data display 4 

MPR trial runs 6 

Monitoring in parallel with fixed RBM system 6 

Independent monitoring 7 

Conclusions 7 

References 8 

ILLUSTRATIONS 

1 . The portable MPR 3 

2. Comparison of events detected with fixed RBM and portable MPR 5 

3. Location of events recorded on MPR over a 1-day period 6 

4. Daily plot of events recorded on MPR over a 1-month period 7 



No 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS 


REPORT 


°F 


degree Fahrenheit ms 


millisecond 


ft 


foot pet 


percent 


h 


hour 





FIELD TRIALS OF A PORTABLE MICROSEISMIC PROCESSOR RECORDER 

By John P. Coughlin and Clinton D. Sines 



ABSTRACT 

The Bureau of Mines has tested a portable microseismic processor re- 
corder at the Galena Mine, Wallace, ID, both in an environmentally con- 
trolled enclosure and, with minimal protection, in a working area of the 
mine. In both trials the device demonstrated effective and reliable 
data acquisition capabilities. 



' Physicist. 

^Mining engineer technician. 
Hazard Detection Group, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



The rate of microseismic activity in a 
mine is related to the ongoing develop- 
ment and relief of stress within the mine 
(3_, _6-_7) • 3 Thus for some years the plot- 
ting and counting of located microseisms 
has provided a general idea of the rela- 
tive stability of specific areas within a 
mine (1). Occasionally these techniques 
have identified an active area immediate- 
ly prior to a rock burst ( 4_) . 

The required techniques and the neces- 
sary hardware for collecting and evaluat- 
ing microseismic data have been detailed 
elsewhere (_L""2_> _5» 8_) . A brief descrip- 
tion of the detection and location of 
microseismic events follows: A transduc- 
er, which may be either a velocity gauge 
or an accelerometer, senses the ground 
motion caused by a microseismic event and 
outputs a voltage that varies with the 
amplitude and frequency of the motion. 
An electronic circuit detects and times 
the initial rise of this voltage above 
background noise. When several trans- 
ducers detect the same microseism, the 
event arrival time at each transducer can 
be used to locate the event (1). 



Fundamentally, microseismic monitoring 
in a mine consists of detecting ground 
motion, locating the associated micro- 
seisms, and plotting the locations or 
energy of the microseisms on mine maps. 
Other, more complex information in the 
microseismic waveform, such as the spec- 
tral energy distribution, could provide 
material for further analysis and perhaps 
enhance the ability of mine personnel to 
evaluate microseismic activity (9^). For 
immediate needs, however, experience sug- 
gests that simple improvements in current 
techniques and equipment would contribute 
greatly to ongoing monitoring efforts. 
These improvements, which are described 
below, relate generally both to the reli- 
ability and accessibility of the micro- 
seismic data and to the ease with which 
mine personnel may install and operate 
the monitoring system. 

This paper describes field trials of a 
portable microseismic processor recorder 
system (MPR) constructed by Science Ap- 
plications, Inc., La Jolla, CA,4 based on 
Bureau of Mines design recommendations. 



GENERAL DESCRIPTION OF PORTABLE MPR 



The portable MPR (fig. 1) provides the 
standard capabilities of an underground 
monitoring device such as a rock burst 
monitor (RBM) (_l.-.2, 5) and in addition 
some new capabilities that enhance device 
utility. As a standard feature, the MPR 
measures the arrival time differences 
among voltage waveforms from a net of 
transducers responding to a microseismic 
event. These time differences, measured 
from the analogue inputs of 16 channels, 
are accurate to within 0.1 ms in a time 
window that adjusts from 1 to 200 ms. A.s 



a second standard feature, the MPR main- 
tains the date in day of year and the 
time in hours, minutes, and seconds and 
provides these data in its output. The 
new and unique features of the portable 
MPR include the portability of the de- 
vice, its mode of storing and providing 
data, its ability to distinguish mine 
drilling noise from real data, and its 
front panel display of event rates. 
These features are described in the fol- 
lowing paragraphs. 



Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



^Reference to specific manufacturers 
does not imply endorsement by the Bureau 
of mines. 




FIGURE 1. - The portable MPR. The five hit-count displays are towards the bottom of the instrument 
face. Above these are the onboard cassette recorder and time-of-day and noise suppression switches. On 
the outside of the case are the 16 water-resistant BNC connectors for analogue input and the waterproof 
connector for the computer interface. 



EASE OF INSTALLATION AND PORTABILITY 

The specific site at a mine where a 
monitoring device (MPR or RBM) is in- 
stalled may influence both the quality of 
the acquired data and the flexibility of 
the monitoring system itself. The loca- 
tion may be either in the mine offices or 
within the mine itself. Installation of 
the system in the mine offices, even if 
they are underground, may require extra 
cable length, which may degrade the 
signal-to-noise ratio and add to mainte- 
nance problems. On the other hand, as 
has been the case with the fixed RBM sys- 
tems at the Galena, installation under- 
ground in the mine itself may require 
construction of special enclosures com- 
plete with air conditioning and ac power. 

These problems , which are inherent in 
choosing a fixed installation site, do 
not arise with the portable MPR, for with 
little or no extra environmental protec- 
tion, the portable MPR will operate near 
the targeted working area of the mine. 
Essentially, all that is required is ac 
power, and should this power be inter- 
rupted, the MPR will maintain the data 
displays and the time of day clock for up 
to 72 h. 

DATA STORAGE AND COMPUTER LINK 

The portable MPR provides data to a 
host computer through a standard RS-232 
interface (fixed at 9,600 baud); alter- 
nately, the MPR will run without a host 
computer and store data in an onboard 
cassette. Commmunication between the MPR 
and the computer is two-way and may be 
initiated by either the computer or the 
MPR. In the former case, the computer 
commands the MPR to transmit a block 
of data for each event stored in inter- 
nal memory. In the latter case, the 
MPR requests permission to begin sending 



data. If permission is not granted, the 
MPR returns to logging in data and stores 
the data in a circular buffer with a 50- 
event capacity. 

In the case of operation without a host 
computer, the MPR simply stores the event 
data on cassette tape. Mine personnel 
may retrieve the cassette and obtain the 
stored data through a cassette reader, 
which outputs either directly to a print- 
er or to a computer. 

Thus the portable MPR logs in data for 
computer analysis whether or not a com- 
puter is immediately available. 

SUPPRESSION OF MINE DRILLING NOISE 

Background mining noise such as drill- 
ing may overload a monitoring system with 
meaningless seismic data and possibly 
prevent detection of both ordinary micro- 
seismic events and rock bursts. Certain 
software techniques can filter out some 
drilling noise (5) , but these techniques 
require a host computer. 

The portable MPR addresses this problem 
without the aid of a host computer. 
Through a system of automatic gain con- 
trol, the portable MPR distinguishes 
drilling from other seismic events and 
eliminates it from further processing. 
Since each channel independently screens 
out drilling noise, one or two overly 
active channels will not cause the MPR to 
suppress genuine microseismic data. 

TOP PANEL DATA DISPLAY 

The portable MPR maintains five dis- 
plays on an inside top panel (fig. 1). 
Four of these displays count the number 
of times a particular geophone has trig- 
gered since the last counter reset; the 
fifth display represents the total number 
of events logged since it was last re- 
set. Mine personnel, knowing where each 



40-287 




40-287 



40-287 





40-287 



FIGURE 2. - Comparison of events detected with fixed RBM (top) and portable MPR (bottom). The 
events were triggered by drilling and bolting in the stope back. Projection A is a side view, and pro- 
jection B is an end view. 



geophone is located, may use these dis- 
play units to get an idea of the spatial 
distribution and intensity of microseis- 
mic events in a given time interval. 
Thus the MPR itself, without additional 



peripherals, provides mine personnel with 
an immediate and useful, though rough, 
summary of microseismic activity through- 
out the monitored area. 



MPR TRIAL RUNS 



The trial runs of the portable MPR at 
the Galena Mine tested not only for the 
reliability and accuracy of the MPR data 
but also for the capacity of the MPR to 
operate with minimal protection in a min- 
ing environment. In the first of two 
sets of trial runs, the portable MPR 
monitored the Galena East end in parallel 
with the well-tested Denver Research Cen- 
ter (DRC) fixed RBM system (_5_ ) . In the 
second set of trial runs, the portable 
MPR independently monitored a stope, 46- 
99, that had suddenly begun showing in- 
tense audible activity. 

MONITORING IN PARALLEL WITH 
FIXED RBM SYSTEM 

Figure 2 shows one day of microseis- 
mic activity recorded on the DRC fixed 



RBM system at the Galena's 40-287 
stope. These events were triggered by 
bolting and drilling for a 6-ft backstope 
round. 

Figure 2 also shows the same activ- 
ity as recorded on the portable MPR. 
The broad features of the two plots are 
the same, but the portable MPR counted 
more events in the same time than did 
the fixed RBM. This count surplus is 
related to the differing storage capaci- 
ties of the two devices. The portable 
MPR, if the host computer or the onboard 
cassette is currently occupied, stores up 
to 50 events in onboard memory. On the 
other hand, the fixed RBM has only 
single-event storage capability and must 
pass this data on to a host computer be- 
fore continuing with data acquisition. 
(The capacity of the fixed RBM system is 



46-87 m 



m 46-99 



~ I 






46-87 



46-99 




FIGURE 3. - Location of events recorded on MPR over a 1-day period. The three circled events are 
of relatively large size and include a small rock burst. Most of the events in the plot followed the burst. 
Projection A is a side view and projection B is an end view of a two-stope system. 



thus a function of the programming within 
the host computer, and in this instance 
the programming is such that the fixed 
RBM system has slightly less capacity.) 

INDEPENDENT MONITORING 

Figure 3 is a plot of 1 day's portable 
MPR data in 46-99 stope. The three cir- 
cled events in this plot were identified 
as relatively large by a software routine 
that checks for triggering in a unique 
low-gain-system channel (_5 ) . The largest 
of these three events was a rock burst 
that registered on a seismograph on the 
surface some 6,000 ft away. Most of the 
other events in the plot followed this 
burst. 

The emplacement of the portable MPR at 
46-99 stope (fig. 3) was part of a prompt 
response to a sudden increase in activity 
around the stope. In 16 work hours, a 
seven-phone network was on-line collect- 
ing data. The portable MPR together with 
a set of variable-gain amplifiers and an 
oscilloscope was installed in a wooden 
box some 300 ft from 46-99 raise. The 
temperature outside the box was 90° F, 
and the humidity was 94° pet. 

Figure 4 is a plot of total activity in 
the stope over a 30-day period. In all, 
the equipment successfully logged in data 
for 60 days, until the low grade of mined 
ore and bursting problems dictated a 
stope shutdown. 




FIGURE 4. - Daily plot of events recorded on 
MPR over a 1-month period. Small rock bursts 
occurred on days 7, 19, and 20. On day 7 most 
of the detected activity followed the burst. 



CONCLUSIONS 



In tests at a working mine, the porta- 
ble MPR has demonstrated the ability to 
provide accurate and useful data. The 
MPR enabled a swift response to an appar- 
ently increasing stress system around a 
stope and operated without failure in 
close proximity to the stope. Moreover, 



for some months after these tests, the 
MPR successfully operated as part of a 
routine monitoring system. Thus the 
portable MPR is demonstrably an effective 
and reliable tool for industry use in un- 
derground monitoring. 



REFERENCES 



1. Blake, W. Microseismic Applica- 
tions for Mining — A Practical Guide (con- 
tract J0215002). BuMines OFR 52-83, 
1982, 208 pp.; NTIS PB 83-180877. 

2. Blake, W. , F. Leighton, and W. I. 
Duvall. Microseismic Techniques for Mon- 
itoring the Behavior of Rock Structures. 
BuMines B 665, 1974, 65 pp. 

3. Brady, B. T. Prediction of Fail- 
ures in Mines — An Overview. BuMines RI 
8285, 1978, 16 pp. 

4. Brady, B. T. , and F. W. Leighton. 
Seismicity Anomaly Prior to a Moderate 
Rock Burst — A Case Study. Int. J. Rock 
Mech. and Min. Sci. , v. 14, No. 3, 1977, 
pp. 127-132. 

5. Coughlin, J. P. Software Tech- 
niques in Microseismic Data Acquisition. 
BuMines RI 8691, 1982, 51 pp. 



6. Leighton, F. W. 
a Major Rockburst. 
1982, 14 pp. 



A Case History of 
BuMines RI 8701, 



7. Obert, L. A., and W. I. Duvall. 
Microseismic Method of Predicting Rock 
Failure in Underground Mining. Part 1. 
General Method. BuMines RI 3797, 1945, 
7 pp. 

8. Redfern, F. R. , and R. D. Munson. 
Acoustic Emission Source Location — A 
Mathematical Analysis. BuMines RI 8692, 
1982, 27 pp. 

9. Rowell, G. A., and L. P. Yoder. 
The Effect of Geophone Emplacement on the 
Observed Frequency Content of Microseis- 
mic Signals. Paper in Third Conference 
on Acoustic Emission/Microseismic Activ- 
ity in Geologic Structures and Materi- 
als - The Pennsylvania State University, 
Oct. 5-7, 1981. 



6U.S. CPO: 1985-505-019/20,043 



INT.-BU.O F MINES, PGH., PA. 27965 



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